1. Field of the Invention
The present invention relates to a flow rate measuring apparatus that measures intake air flow in an internal combustion engine, for example.
2. Description of the Related Art
In conventional flow rate measuring apparatuses, a circuit housing portion in which a sensor body is disposed on a leading end portion is disposed so as to project into a main passage, a passage forming member that forms a bypass passage that bypasses the main passage and allows a portion of intake air to flow through is mounted to a projecting end of the sensor body, and a flow rate detecting element is disposed partway along the bypass passage so as to be electrically connected to a circuit portion that is housed inside a circuit housing portion. The bypass passage includes: an inflow port that has an opening that faces upstream relative to a direction of flow of the intake air on a front surface portion of the passage forming member; an outflow port that has an opening on a side surface of the passage forming member; and a passage portion that bends a plurality of times from the inflow port to the outflow port. A heater is formed on the flow rate detecting element, electric current is supplied to the heater from the circuit portion to generate heat, and the flow rate of the intake air is detected as a change in resistance that corresponds to temperature due to the heater being cooled by coming onto contact with a measured fluid that flows over its surface (see Patent Literature 1, for example).
In flow rate measuring apparatuses of this kind that detect flow rate of intake air as a change in resistance that corresponds to temperature, lean errors in which detected values of the average flow rate of intake air are less than the true average flow rate occur if the flow of intake air pulsates, due to thermal response lag and nonlinear output characteristics of the flow rate detector portion.
In conventional flow rate measuring apparatuses, these lean errors are corrected by using the bypass passage. Specifically, pressure fluctuations occur in the bypass passage that are accompanied by a lag compared to pressure fluctuations at the inflow port and the outflow port of the bypass passage, and lean errors are corrected thereby since the average flow rate of the intake air increases. This effect of increasing the average flow rate of the intake air using the bypass passage is determined by passage length of the bypass passage, length of the main passage between the inflow port and the outflow port, loss in the main passage and the bypass passage, etc., and the effect is also known to be diluted as loss in the bypass passage increases.    Patent Literature 1: Japanese Patent No. 3602762 (Gazette)
In conventional flow rate measuring apparatuses, a bulkhead portion is formed above the inflow port on the front surface of the passage forming member, and blocks the flow of intake air. Thus, flow of a portion of the intake air that has collided into the bulkhead portion above the inflow port is changed toward the inflow port and obstructs the flow of intake air that flows directly into the inflow port from upstream. This obstruction of the flow of intake air leads to a loss in the flow of intake air that flows through the bypass passage.
One problem has been that the flow velocity of the intake air that flows through the bypass passage becomes slower as this loss inside the bypass passage increases, giving rise to desensitization. Another problem has been that disturbances in the flow that are generated as a result of the smooth flow of intake air at the inflow port being obstructed reach as far as the portion where the flow rate detecting element is disposed, lowering measurement accuracy. A further problem has been that the effects that correct the lean errors that occur when pulsations occur in the flow of intake air are diluted as the loss in the bypass passage increases, preventing the lean errors from being corrected satisfactorily.